Found this on the web. not sure if this will help you:
Battery changes (203)
Needed will be a set of leads and a slave battery,
push the red button on the bonnet stay in to open the bonnet to its full
just next to the battery you will find a red cap with a + on it, this is the so call jump point, this is where you would connect your positive lead from the slave battery, on the off side you will find an earth point bolted to the top strut, use this for your earth lead from slave battery, now the car is live, disconnect the positive lead from the battery and tape up fully, this will stop any shorts, where the lead comes thought the plastic, you can lift up the small bit of trim and remove lead right out the way, just be aware that the clips holding this trim are very sharp, then what I do is pull the lead right out the way and tuck it under the brake pipes, hence the tape, disconnect neg lead and battery clamp, replace battery, do not disconnect slave battery until both leads are back onto new battery otherwise damage COULD be caused.
Read more:
http://mbworld.org/forums/c-class-w203/115277-list-faults-occur-after-disconnecting-battery-4.html#ixzz3JRFnUjNe